Process for preserving wood



Patented Apr. 18, 1 939 Ernest R. Boiler, Cleveland Heights, Ohio, as-

signor to E. I. du Pont de Nemours & Company, Wilmington, DeL, a corporation oi Delaware No Drawing. Application April 29, 1938,

Serial No. 205,010

3 Claims. (01. 21-27) This inventionrelates to the preservation of wood and similar fibrous cellulosic materials by impregnation with aqueous solutions containing zincand chromate ions and describes processes for fixing the zinc and chromium in the fibers of the material which are characterized in that the impregnated material, while containing water in excess of the fiber saturation point, is heated under conditions such that a substantial portion of the impregnated material is subjected to a temperature of at least about 220 for at least about one-half hour.

It is generally accepted that an effective wood material treated is more easily obtained. Counteracting the advantages of water-soluble preservatives is the tendency of the material to leach out so that after periods of service the concentration of the preservative in the material treated ma'ybecome lower than the effective toxic concentration. Thus when wood and similar fibrous cellulosic materials are impregnated with aqueous solutions. of zinc chloride and sodium dichromate highly effective preservation is obtained which is subject, however, to the disadvantages incident to the tendency of the zinc and chromium to leach out.

I have now found that the tendency of the zinc and chromium to leach out can be reduced materiallyby heating the impregnated material while containing water in excess of the fiber saturation point under conditions such that a substantial' portinof the impregnated material is subjected to a temperature at least about 220 F'. for at least about one-half hour. In this manner the zinc and chromium are fixed in the fibers by some interaction among the zinc and chromate radicals and unknown organic constituents of the cellulosic material which takes place at elevated temperature in the presence of free water, i; e., water in excess offthe fiber saturation point.

It seems probable that fixation of zinc and chromium by the processes of this invention is related to the phenomena of selective sorption and sludging encountered in the treatment of wood with zinc chloride-sodium" dichromate. When wood is treated with a solution of this preservative, the zinc and chromium contents of the wood are higher than can be accounted for by the amount and concentration of solution retained. There is a fairly constant ratio between the amounts of zinc and chromium thus selectively sorbed. If during the impregnation the temperature becomes too high, a sludge is thrown down which consists of zinc, chromium, and organic matter in fairly constant proportions. It seems 5 likely, therefore, thatby heating the wood after impregnation according to the processes of this invention a similar sludge is precipitated in the fibers. In any event, the tendency of the zinc and chromium to leach out is materially reduced.

In the practical application of my invention wood is impregnated with an aqueous solution of zinc chloride and sodium dichromate according to any suitable impregnation process. Both the full cell and empty cell processes may be used 16 advantageously, tho the full cell process is preferred because the greater amount of free water that remains in the impregnated material is more favorable to the interaction among the zinc and chromate radicals and the organic constit- 20 uents of the wood. The impregnated material, while still wet from the impregnation treatment and therefore containing water in excess of the fiber saturation point, is heated to a temperature at least 220 F. for a suitable period, say one-half hour to one hour or more depending ,upon the sizeof the material treated. Saturated steam at superatmospheric pressure is preferably-used as a heating medium, not only because it is readily available in practically all wood preserving installations but also because, it prevents undue drying during the heating and permits of suitably high temperatures without injury to thewood.

Y The following examplein which the parts are by weight,. is representative and illustrates the effectiveness of the'processes of this invention inreducing the tendency of zinc and chromium to leach out. v

Example Thirty-six blocks of short leaf pine K- 2nx1]/2n 11/z! rates of leaching are shown by the following These data show clearly that leaching of the zinc and chromium is materially reduced, the zinc remaining in the treated blocks being about 2.6 times that in the untreated blocks and leaching of chromium being prevented entirely.

While the' above example sets forth particular conditions it will be understood that they are merely illustrative and that numerous variations may be made therein without departing from the spirit of the invention, as long as the impregnated material while containing water in excess of the fiber saturation point is heated under conditions such that a substantial portion of the impregnated material is maintained at a temperature of at least about 220 F. for at least about one-half hour.

'These two factors are not only interrelated but are dependent upon the initial temperature and besides on the kind of wood being treated. It is obvious that a longer time will be required to heat a large piece ,of timber than a small piece. In the same manner the higher the initial temperature the shorter the time that will be required to raise the impregnated material to the desired temperature. It is also much easier, for instance, to fix the preservative in Southern yellow pine than in hard wood such as oak. In any case, however, the impregnated material should be held at a temperature of at least about 220 F. for at least about one-half hour.

While those skilled in the art with a little experience will be able to select suitable temperatures. depending upon the size of the impregnated material and the kind of wood impregnated, it is generally desirable to work at the highest temperature possible without injury to Wood since higher temperatures will permit a shorter duration of the treatment. Yellow pine, for example, may be subjected to a steam pressure of about twenty pounds per square inch gage (260 F.) for a considerable time without any deleterious effect. Even higher temperatures, such as those corresponding to about 25 pounds per square inch steam pressure (266 F.). may be used with little if any'deleterious effect. For most purposes I have found it desirable to hold the impregnated material at a temperature between about 240 F. and 260 F. for from about one-half hour to one hour. In the case of any sizable piece of timber it'will be sufficient, for example, to steam the wood for a time necessary to raise the temperature at the center of the piece from about 240 F. to about 260 F. It will be understood, however, in view of the complexity of the factors involved that the treating conditions may be varied widely and that those given of wood, from about 28 to 35 per cent moisture figured on the basis of the dry weight of the wood. As a practical matter the moisture content of the wood will be satisfied if seasoned wood is impregnated with preservatives and thereafter heat treated as described.

While the most satisfactory results are obtained by the use of steam as the heating medium, my invention is not limited in this respect since any heating media suitable for heating wet wood to the proper temperature and for the proper time is suitable. As a practical matter, however, heating media such as dry hot air or similar gases, which at the specified temperatures would cause rapid loss of water from the outside portions of the wood, should be avoided since the resulting shrinkage in connection with the swollen condition of the inside portion of the wood might tend to cause checking and warping. Ordinarily, however, the treatment need not be continued long enough to give rise to these conditions but, should it be desirable to continue the heating for a longer period, excessive loss of water from the outside portions of the wood may be avoided by saturating such a heating media with water vapor or 'by effecting the heating in a closed retort which would restrict excessive evaporation of water. In some cases it may be advantageous to carry out the heating in an oil bath. Such a treatment ofiers advantages in that by the application of the proper pressure the wood may also be impregnated with a suitable preservative oil.

The processes of this invention are particularly applicable to the treatment of lumber, timtageous to avoid heating under conditions conducive to excessive drying since the felted material containing zinc chloride in solution in the fibers is subject to charring during drying at excessively high temperatures.

While I have discussed principally zinc chloride-sodium dichromate preservatives it is to be understood that this is merely representative and that other soluble zinc salts and other soluble dichromates may be employed if desired. The only essential criteria insofar as the preservative of the impregnation step is concerned are the. presence of zinc and the presence of chromate ions in aqueous solution.

I claim:

1. In a process for the and similar fibrousv cellulosic materials by impregnation with aqueous solutions containing zinc and chromate ionsthe method for fixing the zinc and chromium in the fibers of the material which comprises heating the impregnated material while containing water in excess of the fiber saturation point under conditions such that a substantial portion of the impregnated material is heated to a temperature of at least about 220 F. for at least about one-half hour.

2. In a process for the preservation of wood and similar fibrous cellulosic materials by impregnation with aqueous solutions containing zinc and chromate ions the method for fixing the zinc and chromium in the fibers of the material which comprises heating the impregnated material while containing water in excess of the fiber saturation l preservation of wood point under conditions such that a substantial portion of the impregnated material is maintained at a temperature between about 240 to 260 F. for at least about one-half hour.

3. In a process for the preservation of wood and similar fibrous ceiiulosic materials by im-' pregnation with aqueous solutions containing zinc and chro'mate ions the method for fixing the zinc andehromium in the fibers o! the material which comprises subjecting the impregnated material to heat treatment with saturated steam at superatmospheric pressure at a temperature between about 240 F. and about 260 F. for at least about one-half hour.

ERNEST R. BOLLER. 

